The present invention relates to a system and method for sampling a gas feed stream to determine the presence and characteristics of particulate contaminants in the gas stream. In particular, the present invention relates to a system comprising a particle counter and a particle capture filter, wherein the particle capture filter is advantageously arranged in parallel with the particle counter.
Many users of specialty gases, such as semiconductor device manufacturers, require low suspended particle content in the gases. For example, particulate contamination in fabricating materials causes low yield in the device fabrication process, and reliability problems in finished semiconductor devices. Therefore, strict cleanliness requirements are routinely imposed upon gases such as, but not limited to, Ar, He, N2, Xe, Kr, Ne, SiH4, SiH2Cl2, NH3, BCl3, CO2, CO, N2O, O2, H2, SiHCl3, PH3, AsH3, BF3, B2H6, Si2H6, SiCl4, and many others.
Particulate levels in gas feed streams may vary between being a relatively uniform and steady stream or, for example, as the tool interface is approached, variable over a period of time. Variability in the gas feed stream can take the form of burst states (spikes), time-varying drift (upward or downward), and/or step changes (upward or downward). In dynamic systems, such as, for example, flowing transfill systems and tool feed lines, the gas feed stream is usually well mixed and particles are uniformly distributed. However, in static systems, such as, for example, gas cylinders or other supply vessels, particulate levels can vary spatially by orders of magnitude. This particle variation may be attributable to such effects as gravitational settling and diffusion to internal surfaces. Such effects produce non-uniform particulate distributions, including stratification, in the supply vessel.
Cylinder and bulk gases are frequently reduced in pressure with an automatic regulator before entering the gas feed stream. This reduction in pressure of the gas feed stream may produce, for example, increased particulate levels through regulator “shedding”, impurities nucleation, and condensational droplet formation. In certain situations, suspended non-volatile residue formation may occur.
In addition to the above issues, if the gas feed stream comprises a reactive gas such as, for example, silane, the reactive gas may combine with atmospheric contaminants to form suspended solid material (particles). Reaction of silane with oxygen or oxidizing agents produces silica (SiO2) dust in the form of particles. Any trace moisture or oxygen in silane storage/transfer systems can be expected to produce copious amounts of fine particulate silica. These solid reaction products can produce a significant inaccuracy in any measurement of the suspended particle content. Such particulate generation persists until the oxygen or oxidizing agents in the system are consumed and the source of the agents is eliminated. Because of these and other issues, careful attention to the detection and removal of atmospheric contaminants may be necessary for gas feed streams comprising reactive gases.
Particle formation in a gas feed typically often results from the presence of molecular impurities. Many semiconductor processing gases are supplied in pressurized vessels. It is common for such high purity gases to contain trace molecular impurities, such as, for example, hydrocarbons in nitrogen, siloxanes in silane, and other such impurities depending upon the composition of the high purity gas. These impurities may result from the processes used to produce, transfer and store the gases in pressurized containers. The internal pressure and temperature of the gas storage vessel are frequently well above the critical point pressure and critical point temperature of the gas. For example, the critical points of N2 (492 psia, −232° F.) and SiH4 (703 psia, 26° F.) are typically exceeded in gas storage vessels as delivered to users. It is well known that supercritical fluids have a high solvent power for materials, such as higher molecular weight hydrocarbons, which may exist as surface contaminants in gas transfer, storage and delivery systems. These dissolved impurities may add to the molecular impurities typically present in the gas.
In order to control process variables that contribute to particulate contamination in gases and to ensure the quality of the gas, accurate particle measurement from pressurized gas sources is performed. It is desirable to measure the suspended particle concentration in the pressurized gas. However, due to the pressure limitations of available instrumentation, it may not be practical to measure the particle content at the full pressure of the storage vessel. Consequently, the gas sample is transferred through a pressure reducing device, such as, for example, an automatic pressure regulator, valve, flow restricting orifice, or the like, in order to reduce the gas pressure to a level compatible with the available instrumentation for particle measurement. This measurement may be conducted in-line or off-line relative to the gas feed stream.
It is well known in the art of particle measurement that gases having trace quantities of molecular impurities suffer an increase in particle content as the gas pressure is reduced. This degradation results from molecular clustering of trace impurities leading to formation of stable (i.e., persistent) suspended particles. These particles cannot be easily vaporized through heating. Further, in certain instances, the process of pressure reduction frequently produces sub-critical conditions in the gas. In this regard, the sub-critical gas loses its high solvent power following pressure reduction. Any dissolved impurities therefore tend to form stable suspended particles in the sample gas stream. Particle formation during pressure reduction is known to produce particles levels of over 106 per standard cubic foot of gas for particles larger than 0.02 micrometer. This level substantially exceeds the actual level of particles in the pressurized vessel. FIG. 1 provides an example of a typical gas feed stream 1 that is passed through a pressure reducing device such as valve 2 that is in fluid communication with gas feed stream 1. While gas feed stream initially contains low levels of gas-borne particles 3, after passing through valve 2, the amount of particles contained within gas feed stream 1 or “nucleated” particles 4 increases. These nucleated particles 4 within lower pressure gas stream 5 are carried to the downstream particle counter instrument (not shown). The actual particle concentration in the vessel cannot then be discerned from the measurement. The pressure reduction process therefore substantially degrades the accuracy of particle measurement.
Previous attempts to solve the problem include construction of pressure resistant particle counters. Such instruments eliminate the need for sample gas pressure reduction upstream of the instrument. These instruments, however, cannot provide information on composition and morphology of the measured particles.
Similarly, low pressure instruments may be placed in custom-built pressurized chambers (e.g., hyperbaric chambers) and operated near the source gas pressure. However, this is an expensive and difficult modification to the instrument design and therefore suffers from practical problems.
Pressurized filter devices may also be used to capture particles from the sample stream without prior pressure reduction. These captured particles can then be examined using various means, such as optical microscopy, scanning electron microscopy, digestion or dissolution in liquid media followed by compositional analysis of the liquid, etc. However, this test method cannot discriminate between particles originating in the gas supply source and those spurious particles formed in the sampling system.
The problematic molecular impurities in the pressurized gas may be removed prior to pressure reduction using various absorbents, adsorbents, catalytic purifiers and other devices well known in the art of gas purification. This method has been known to substantially reduce or eliminate particle formation during pressure reduction. Such purifiers, however, operate by passing the gas feed stream through a bed of granular or pelletized purifying medium. This bed would tend to also act as a filter to remove actual particles flowing from the pressurized vessel or may introduce new particles to the stream from the purifying medium. Therefore, the actual particle content in the pressurized vessel cannot be accurately determined downstream of a bed-type purifier.
Gas stream heating and heating of the pressure reducer device have been used in an attempt to prevent nucleation of impurities during pressure reduction. This method is usually not effective in preventing the formation of nucleated particles in the expanded gas stream.
Accordingly, there is a need in the art for an improved, reliable system for measuring and/or analyzing particles within a gas feed stream that effectively removes molecular impurities before such impurities contribute to false impurity measurements.